A method for preparing glassy borate disks for instrumental analysis and in particular for x-ray fluorescence analysis is known in which a determined amount of the investigated material and a borate flux is melted in platinum crucible or in a crucible made of platinum admixed with an other noble metal--gold, rhodium etc. The melt thus obtained is poured in moulds made also of platinum or a mixture of platinum with an other noble metal amd afterwards the melt is cooled stepwise at temperatures 400 and 200 degrees C. See, Afonin V. P., Gunicheva T. N., X-ray Spectral Fluorescence Analysis of Ores and Minerals, Publ. House "Nauka", Moscow, p.188-211,208; Kawasaki Steel Corp. JP 80, 151, 248; Nippon Steel Corp. JP 58, 123, 441; and Nippon Steel Corp. JP 60, 27, 817.
The above mentioned known method has the following disadvantages: the process of preparing glassy disks is very labour consuming and has a low productivity as the preparing of the melt and its pouring in the moulding vessels proceeds singly and consecutively. A part of the melt sticks to the crucible and it should be removed by melting with a cleansing means (borate, sodium iodide, lithium fluoride etc.) or by boiling off in concentrated hydrochloride acid. Moreover, the crucibles and the moulding vessels are manufactured using costly and deficient materials.
Another method for preparing glassy borate disks for x-ray fluorescence analysis is known in which the mixture of the investigated material and borate flux is melted in crucibles made of graphite or glass-graphite and afterwards the melt is poured in moulding vessels also of graphite or glass-graphite and then it is cooled. See, Maljutina, T. M. et al, ZH Anal. Himija, 1983, NR. 12, V.38, p.2137-42.
The drawbacks of this second method are the following: the process is also very labour consuming; the castings are obtained after pouring the melt from the crucible in the moulding vessels that is carried out singly and consecutively; in the case of graphite crucibles, a large amount of graphite in the melt is penetrating thus contaminating it; due to filling up only of a small part of the crucible volume with melt it burns fast; the crucibles made of glass-graphite can not endure a temperature higher than 800 degrees C. so that the use of such crucibles of this type for temperatures higher than 800 degrees C. is possible only once and there is a risk of melt running out. The inconveniences mentioned hereinabove are not avoided and in making, based upon these two methods, semi-automatic and automatic apparatus for preparing glassy disks for x-ray fluorescence analysis, the apparatuses also have a high price and need a continuous operational maintenance.
A method for preparing glassy borate disks for x-ray fluorescence analysis is known in which the mixture of the investigated material and the flux is homogenized, pressed, and the thus obtained tablet is melted in a shallow graphite crucible in a muffle furnace. Afterwards, there is provided a vacuum device for removing all gas inclusions from the melts. See, Bowling, Gerald D., Ailin-Pyzik, Iris B., Jones David R., IV Adv. X-ray Anal., 1984, 27, p.491-96.
This last mentioned method has the following draw-backs: only the central part of the surface of the prepared glassy disks is used in the x-ray fluorescence analysis due to contamination of its periphery by graphite inclusions; for each separate melt a special vacuum device is used for removing the gas inclusions from the melt in the graphite crucible; in case of simultaneous preparation of more than one melt, the number of the required vacuum devices has to correspond to the number of melts.
Another method for preparing glassy borate disks for x-ray fluorescence analysis is known in which the mixture of the investigated material and the borate flux is homogenized, tableted, and melted in graphite rings that are placed on a graphite support at temperatures 1000 to 1100 degrees C. with following cooling of the melt in a second furnace that has been in advance heated to a temperature of 600 degrees C. The cooling is performed gradually to room temperature for a period of 6 to 7 hours. See, Bulgarian Inventor's certificate BG 28362.
The draw-backs of this method are: need of using a second furnace; long periods of cooling of the melt; necessity to grind and polish the glassy borate disks in order to remove the graphite inclusions from the surface used for x-ray fluorescence analysis.
A common inconvenience of all methods mentioned hereinabove is that they do not permit a simultaneous preparing of a large number of glassy borate disks for instrumental analysis.